Laser is an acronym for “Light Amplification by Stimulated Emission of Radiation.” The term laser refers generally to a category of optical devices that emit a spatially and temporally coherent beam of light otherwise known as a laser beam. There are a variety of different types of lasers, such as electrically-pumped semiconductor lasers, optically-pumped solid-state lasers (e.g., a laser using a sapphire crystal as a lasing medium), and optical fiber lasers.
Lasers have found great utility in a number of different applications, such as optical communications, welding, materials processing, and surgical applications because of the unique optical characteristics of a laser beam. For example, a laser beam typically exhibits one or more of the following desirable characteristics: (1) a very narrow bandwidth; (2) an ability to propagate over long distances without significant divergence; and (3) and an ability to be focused to a very small beam diameter.
In many applications, it is desirable to increase the power of a laser beam by power scaling. Power scaling entails combining the output power of multiple individual laser sources into a single high-power beam of beam quality comparable to that of the individual laser sources.
One power-scaling technique for combining multiple high-power laser beams is spectral beam combining, which is also commonly known as wavelength beam combining, wavelength combining, or incoherent beam combining. In spectral beam combining, respective high-power laser beams with non-overlapping optical spectra are combined using a spectral beam combiner (also known as a wavelength-selective beam combiner), such as a prism, or more commonly, a diffraction grating. The spectral beam combiner directs each respective high-power laser beam according to their peak wavelengths so that the directed laser beams propagate in generally the same direction as a single, substantially spatially-coherent beam.
While spectral beam combining is an effective beam combining technique, it suffers from an intrinsic limitation with regard to scalability. The number of laser beams of different peak wavelengths that can be spectrally combined is determined by the bandwidth of the laser gain medium (e.g., 1045 to 1090 nm for a typical Yb-doped fiber amplifier) and the minimum allowable spacing between adjacent wavelengths. The minimum allowable spacing between adjacent wavelengths is determined by the resolving power of the wavelength-dispersive element of the spectral beam combiner and the physical size of the spectral beam combiner. For example, if the minimum allowable wavelength in a spectral beam combiner is 10 nm, then five separate laser beams output from respective Yb-doped fiber lasers operating at 1050, 1060, 1070, 1080 and 1090 nm can be spectrally combined into a single, spatially-coherent laser beam. However, the resulting combined laser beam is unsuitable for further spectral beam combining. For example, two identical spectrally-combined laser beams cannot be spectrally beam combined because they are not spectrally distinct. In addition, it would not be possible to spectrally beam combine such a spectrally-combined laser source with a second spectrally beam combined source having a spectrum including intermediate wavelengths of 1045, 1055, 1065, 1075, and 1085 nm because the minimum wavelength spacing specification of 10 nm would be violated. Thus, it is not possible to perform multiple stages of spectral beam combining. Furthermore, the number of emitters that can be combined using spectral beam combining is limited accordingly.
Coherent beam combining is another technique for combining multiple laser beams. In coherent beam combining, multiple high-power laser beams can be combined into a single laser beam using the well-known side-by-side combining and filled-aperture combining techniques. While coherent beam combining preserves the spectral bandwidth from the multiple high-power laser beams, the technique requires precise control of the phase and polarization of each of the multiple high-power laser beams. Therefore, laser apparatuses employing coherent beam combining can be very complicated and difficult to operate efficiently in practical applications outside of a laboratory.